Electromagnetic radiation has been used in a wide array of noninvasive diagnostic applications. X-rays have been used for many years to create a two dimensional image of the inside of an object. Computed axial tomography scanners are able to generate three dimensional images from a series of two dimensional x-ray images. Magnetic resonance imaging (also known as nuclear magnetic resonance spectroscopy), such as disclosed in Harms et al., U.S. Pat. No. 5,415,163 A and Rapoport et al., U.S. Pat. No. 4,875,486 A, operate by first applying a magnetic field to a subject so as to align, in a uniform manner, the nuclei of atoms within a portion of the subject to be tested. These aligned nuclei are then briefly exposed to a radio frequency (RF) signal set to a specific frequency, which causes each of the various aligned nuclei at a lower energy state to spin or flip to a higher energy state, known as a resonant frequency. The magnetic field is then removed or altered, causing the nuclei forced to a resonant frequency to become unstable and return to their original lower energy state. This later process is called spin relaxation. The faint energy released from the spin relaxation is then collected as a representation of the nuclei within the sample.
Hence, the spin relaxation energy released by the sample is used to generate an image that is representative of the sample. The RF signal itself is not utilized for detection or imaging purposes—it is only used to excite the nuclei to a higher energy state and is removed before the spin relaxation energy is detected. Further, the magnetic field(s) are only used to align and then release the nuclei in the sample, and are removed or altered before spin relaxation can occur.
While electromagnetic signals transmitted through a specimen have been used to detect or measure the concentration of various chemicals in that specimen, such prior techniques were not highly accurate and results were often difficult to repeat. For example, U.S. Pat. No. 4,679,426 disclosed a non-invasive technique for measuring the concentration of chemicals, such as sodium chloride, in a sample. Periodic electromagnetic waves between 10 MHz and 100 MHz were coupled to a subject's finger and resulting waveforms were found to be indicative, at specific frequencies (i.e., 17.75 MHz for sodium chloride and potassium chloride), of concentration levels of those chemicals in the finger. Likewise, U.S. Pat. No. 4,765,179 used periodic electromagnetic waves between 1 MHz and 1 GHz, that were coupled to a subject's finger, to generate a waveform that provided meaningful analysis of glucose levels in the subject based on the presence of other compounds in the subject's blood at specific frequencies (i.e., 17.75 MHz for sodium chloride and potassium chloride, 11.5 MHz for ethyl alcohol, etc.).
In U.S. Pat. No. 5,508,203 (the “'203 patent”), high frequency electromagnetic radiation was coupled to a specimen through a probe pair to generate a signal of varying amplitude or phase that could be compared to a source signal to determine the presence of a target chemical, such as NaCl, to help determine glucose levels. While this later technique represented an improvement over the prior methods, it was soon realized that electrolytes, e.g., NaCl, KCl, NazHPO4, and KH2PO4 of varying concentrations in human blood, can affect the accuracy of glucose measurements using the '203 patent.
To account for the deficiencies in the '203 patent, a new technique was developed in U.S. Pat. No. 5,792,668 (the “'668 patent”), in which two signals were transmitted through the subject at the same time and the magnitude of impedance at the subject was measured to determine a glucose level in the subject. In particular, the two signals had a cross-over frequency of about 2.5 GHz that provided the best measurement of impedance. In blood specimens, it was found that electrolyte concentration effects are effectively “tuned out” by examining impedance at this cross-over frequency. A similar approach was applied in U.S. Pat. No. 7,184,810 (the “'810 patent”), which cites the '668 patent. In the '810 patent, a probe is applied to the subject's skin, through which electric pulses from a pulse generator are fed and partially reflected back to a measuring device, where a time resolved measurement is made. The glucose level is determined from matching the measured voltage to a calibration table.
The next evolutionary step in the development of electromagnetic energy signals to determine the presence and concentration level of chemicals within a subject is represented in U.S. Pat. No. 6,723,048 B2 (the “'048 patent”), which is assigned to the assignees of the present application and which discloses a noninvasive apparatus for analyzing blood glucose and similar characteristics. The '048 patent apparatus utilizes spaced apart transmission and detection nodes placed on either side of and in contact with a sample to be tested. The nodes are always in close proximity to one or more pairs of magnets that create a magnetic field that envelopes the nodes and the sample between the nodes. An RF signal having a frequency between 2 GHz and 3 GHz is transmitted from the transmission node through the sample and to the detection node.
The detected signal is then sent to an analyzer that employs pattern recognition techniques to compare the detected signal at a specific frequency (with respect to glucose, the '048 patent specified 2.48 GHz), to previously detected signals at the same frequency to make a determination regarding the characteristic of the sample being tested. For example, if the sample was a finger of a patient that had previously been tested when the patient was known to have different glucose levels (verified through a more traditional form of glucose testing) to create three or more previously detected signal patterns, the presently detected signal would be compared to each of these previously detected signal patterns to determine which pattern it most closely resembled in order to approximate the patient's present blood glucose level.
In addition to testing glucose levels and other blood chemistries, it has been speculated that electromagnetic frequency spectrum technologies could have application to the biometric identification field, but no reliable technique has been developed for this purpose. In many fields of activity, it is essential that persons be identified or their claimed identity be authenticated. Examples of such fields include granting physical access or entry into buildings, rooms or other spaces, airport security, credit card purchasers, ATM users, passport verification, electronic access to information or communication systems, etc.
A number of noninvasive detection technologies have been developed to address these needs, such as fingerprint scans, iris and retina scans, and voice recognition. These technologies operate on the principal that individuals possess unique and unchanging physical characteristics that can be measured and compared with stored data. The basic requirements for acceptable biometric technology are that it must allow for practical widespread use, be accurate and reliable, be difficult or impossible to circumvent, be quick, easy and convenient, present no or little privacy violation concerns, be low cost to produce, and be consumer friendly. Current biometric identification and authentication technologies do not meet all of these basic requirements.
Iris and retina scanning technologies can be highly accurate, but the equipment used in scanning is expensive and requires substantial space. Further, humans are highly uncomfortable with the idea of having their eyes scanned with a laser or infrared light or having their picture taken and stored by a machine (and then used by who knows who). Also, iris and retina scanners have been spoofed with a number of techniques that have required the technologies to be modified in various ways, making the technology more expensive, less convenient, and less consumer friendly.
Electronic or optical fingerprint scanning systems are inexpensive, but are not very accurate, are easily damaged, and can be easily spoofed. Variations in skin, ethnic races with very light fingerprint patterns, people with unusually dry skin, elderly people, people with rough hands, water webbing, abrasions and cuts have all been known to create difficulties for fingerprint systems. Furthermore, many people consider fingerprinting to be an invasion of their privacy because of the heavy use of fingerprinting for law enforcement purposes. Additionally, many fingerprint scanning devices have been easily spoofed with objects as common as gummy candy.
Voice recognition systems tend to be the least accurate of the other biometric identification and authentication technologies. Voices can be readily recorded or mimicked, and allergies, colds and other respiratory issues can readily produce false negatives. Hand geometry and face recognition systems suffer from similar issues. They also tend to require a large amount of space and face recognition systems can be expensive. As with fingerprints, changes in a subject's skin, such as a suntan, a burn or a skin condition, or other changes to a subject's physical appearance can present problems for the system.